Silicon nitride kiln furniture

ABSTRACT

A device or item of kiln furniture for the conveyance of silicon wafers into and out of a diffusion furnace in the manufacture of silicon diodes. The device is made essentially of high purity silicon nitride and consists of an elongated base with rotatable wheels attached thereto, and supporting means for carrying silicon wafer laden silicon nitride or silicon boats. The device, by virtue of its design and material of which it is composed, satisfies the peculiar and stringent requirements of silicon diode manufacture, by virtue of the devices ability to retain its integrity through repeated thermal shock, its chemical purity, and its non-dust generating character.

[ Dec. 12, 1972 Edited tates Patent Alliegro [54] SILICON NITRIDE KILN FURNITURE Primary Examiner-Charles J. Myhre AttorneyRufus M. Franklin [72] Inventor: Richard A. Alliegro, Holden, Mass. [73] Assignee: Norton Company, Worcester, Mass.

[22] Filed:

May 20, 1971 ABSTRACT A device or item of kiln furniture for the conveyance of silicon wafers into and out of a diffusion furnace in the manufacture of silicon diodes. The device is made essentially of high purity silicon nitride and consists of an elongated base with rotatable wheels attached thereto, and supporting means for carrying silicon wafer laden silicon nitride or silicon boats. The device,

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[5 References Cited by virtue of its design and material of which it is composed, satisfies the peculiar and stringent requirements of silicon diode manufacture, by virtue of the devices ability to retain its integrity through repeated thermal shock, its chemical purity, and its non-dust generating character.

2 Claims, 4 Drawing Figures 3,570,827 3/1971 Breedon 2,776,131 1/1957 Marino, Jr... 3,604,694 9/1971 Muller........

PATENTEnuEc12 I972 3.705714 VENTOR. LL I EGRO ATTORNEY RICHARD A X SILICON NITRIDE KILN FURNITURE BACKGROUND OF THE INVENTION diffusion The general process for the manufacture of silicon semi-conductor components such as diodes and transistors, involves oxidation of thin silicon wafers for electrical insulation, etching of cavities in the surface of the wafers, and the deposition of boron, phosphorous, arsenic, or antimony into the cavities which function as the transistor contact points. These semi-conductor components are made separately or in an integrated circuit array. The oxidation step and subsequent diffusion operation to which the silicon slices are subjected to, involve a rapid heat and cool cycle in furnaces at temperatures in the range of from l000 to 1325 C. This critical heat treatment generally takes place in an electrically heated wire-wound muffle furnace. The silicon slices are placed on quartz (silica) or silicon kiln furniture which is then placed within the work tube of the muffle furnace and the silicon slices are fired through a predetermined temperature-time cycle. The diffusing agent is usually introduced in the form of a gas which is fed into the necked-down end of the difusion furnace work tube. The gas then diffuses in the cavities which had previously been etched into the surface of the slices or wafers. In the oxidation and diffusion steps of the process the silicon slices are placed on boats or plates which are supported on appropriately designed kiln furniture. The kiln furniture must be made of a material with excellent thermal shock resistance in order to permit rapid heating to and rapid cooling from temperatures in the order of lOO to 1325 C and back to room temperature. The material of which the kiln furniture is constructed must also be of high strength, have the ability to retain its shape despite a great number of heating and cooling cycles, the material must not out-gas i.e., introduce any undesirable impurities into the atmosphere of the kiln during firing operation, and the kiln furniture must not introduce any dust-like contamination.

These ultra stringent conditions severely limit the materials which can successfully be used to fabricate kiln furniture. The kiln furniture presently in use is made of quartz, however the quartz furniture suffers from two serious shortcomings, namely, the loss of strength and progressive devitrification with time at temperatures above 1200 C. Further, quartz kiln fumiture is very susceptible to severe distortion from frequent heating and cooling of the material.

If the kiln furniture is constructed of moving parts then the frictional characteristics of the material at elevated temperatures must not result in the generation of any dust contamination, and the friction between the moving parts must be sufficiently low to facilitate a smooth entry and exit from the diffusion furnace.

An object of the present invention is kiln furniture for the production of semi-conductor components said kiln furniture having the characteristics of; oxidation resistance, superior thermal shock resistance, high strength, ability to retain its shape after a large number of heating and cooling cycles, no out-gassing at the temperatures of the process, and no dust generation by the moving parts.

SUMMARY OF THE INVENTION The invention is a wheeled item of kiln furniture for use in the manufacture of diodes and transistors, which is made up essentially of silicon nitride. The invention item of kiln furniture is assembled from parts which have been hot-pressed from powdered silicon nitride containing from 0.5 to 5.0 percent by weight of magnesium oxide, the latter material being essential to the hot-pressing of high density silicon nitride. The magnesium oxide is inert toward the silicon wafers being processed to semi-conductor components. The present item of kiln furniture is constructed of a base member, at least one set of rotatable wheels and supporting members attached to said base, the latter for the purpose of supporting silicon or silicon nitride boats or batts which in turn carry the silicon wafers which are to be processed. The wheeled kiln furniture of the present invention, by virtue of its being composed essentially of silicon nitride, successfully overcomes the shortcomings of the prior art quartz furniture, viz. severe distortion and devitrification resulting in structural weakness.

Furthermore, because of its stability in the semi-conductor manufacturing process, the silicon nitride kiln furniture is readily amenable to automatic drive systems for cycling the silicon wafers in and out of the diffusion furnace. Automation of the process results in improved reproducability of desired properties in the semi-conductors, improved efficiency, and process cost reduction.

I BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a wheeled silicon nitride item of kiln furniture with silicon wafer laden boats resting on the support members thereof.

FIG. 2 is a perspective view of a silicon nitride item of kiln furniture of a different construction.

FIG. 3 is a perspective view of that part of a support member which is in direct contact with and retains the boat.

' FIG. 4 is another embodiment of that part of the support member which is in contact with the boat.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The silicon nitride of which the kiln furniture of the present invention is fabricated, is synthesized from extremely pure, electronic grade silicon of at least 99.99 percent purity. The silicon is nitrided at elevated temperature with equally pure nitrogen to yield the compound Si N The degree of purity in the base material is a requisite to the production of kiln furniture that will not out-gas impurities during the oxidation and highly critical diffusion steps of the silicon semi-conductor manufacturing process.

The pure silicon nitride is then ball milled to reduce the material to a powder, followed by acid treating and washing of the powder to remove any iron contamination resulting from the milling process. An alternate method involves the use of jet-milling techniques.

The silicon nitride powder is then used to form the components of the kiln furniture viz. base, wheels, and supporting members. Preferably this is done by hotpressing refractory shapes is well-known and its technology is incorporated herein by reference. In order to hot-press the silicon nitride parts to optimum specific gravity, for the sake of maximum strength of the final product, a small quantity (0.5 5.0 percent) of magnesium oxide must be blended with the silicon nitride powder. The magnesium oxide present in the final item of kiln furniture is inert during the silicon semi-conductor manufacturing process. The hotpressed parts are then machined or ground to the exact dimensions desired and they are then assembled into the final wheeled conveyor for the silicon wafer laden boats or plates. The wheeled conveyor may be assembled from its parts completely through mechanical interlocking of said parts or by adhering the parts with silicon nitride cement, or a combination of these. The various permutations and the scope of the present invention will be more readily understood by a consideration of the attached drawing.

FlG. 1 illustrates a silicon nitride wheeled conveyor with boats 22 laden with silicon wafers 26. The conveyor is made up of a base member 12 which extends the full length of the device, and which may be further extended to form a handle 28. Attached to each end of the base member 12 is an axle and a set of rotatable wheels 14. The bearing surfaces of said axle and wheels are ground and polished to minimize friction between them thereby facilitating a smooth entry and exit from the tube furnace, and to eliminate the generation of dust by these bearing surfaces. Also affixed to the base member 12 are appropriately spaced support members 16 which contain notches in the upwardly extending appendages thereof, for the purpose of holding the boats 22 in place with respect to any movement in the direction of travel of the conveyor as it enters and exits from the furnace. Also affixed to the base 12 by silicon nitride cement, are the positioning or retaining members 18 which serve, in combination with the slots 24 in the boats 22, to prevent movement of the boats or plates in a transverse direction. The cementing is preferably affected by applying to the surfaces to be adhered, a paste of powdered silicon metal and an aqueous colloid of silica, followed by firing in a nitrogen at- I mosphere thereby drying and converting the silicon-silica paste to silicon nitride. The cement, however, need not necessarily be silicon nitride. Applying a paste of a silica containing material like ethyl silicate for example, followed by firing in an oxygen atmosphere provides a suitable cement. This particular construction may be of any desired length, to carry a multiplicity of boats. A longer conveyor is readily adapted from a longer base and additional support members, retaining members, and rotatable wheels.

FIG. 2 is another embodiment of the present invention. It differs from the wheeled conveyor of FIG. 1 primarily in that the various components of the device are assembled strictly by mechanical interlocking of said components. The base member of the device 32 is made up of two parts 34 and 36 of such a configuration that when mated form round passages 42 through which pass the wheel axle, and angular passages 43 through which pass to support members 44 and 48. The ends of the two parts 34 and 36 of the base member are notched in a manner such as that shown. A cap 38 is fastened to fit tightly over said notched ends thereby holding the entire assembly together. The cap on one end may be elongated so as to provide a handle for moving the wheeled conveyor in and out of the furnace. The two halves 34 and 36 of the base member are assembled with the support members 48 and 44 locked between them as shown. The support members 44 are similar to those in FIG. 1 identified as 16, and having a notched end in the upwardly extending appendages thereof, which serve to retain the ends of the boats. Support member 48 is an intermediate support and need not have any notch on the ends of the upwardly extending appendages. Pins 46 are located in holes provided therefor in the upper half 34 of the base member; these pins prevent transverse movement of the boats with relation to the wheeled conveyor, by mating with holes provided therefor in said boats. With this pin arrangement the aforementioned notches on the upper ends of the support members 44 are not necessary to prevent movement of the boats; i.e., the upper ends of support members 44 could be the same as the upper end of support member 48. Similarly, the axles 41 and wheels 44), which are attached to each other by a press fit or with a hole and pin arrangement, are located in between the two halves 34 and 36 of the base. Here the axles turn in the holes 42 therefore holes and axles should be ground and polished to reduce friction and dusting. The various interlocking components are then fixed in position by the caps 38. All of the parts are formed from high purity silicon nitride, and preferably by hot-pressing which requires the use of from 0.5 to 5.0 percent by weight of magnesium oxide in the silicon nitride. I

Various design innovations are possible and are considered within the scope of the present invention. For example, the ends of the upwardly extending appendages of the support members 16 and 44 shown in FIGS. l and 2 respectively could be modified as shown in FIG. 3. in such a design there would be no need for the positioning members 18 and pins 46 in FIGS. 1 and 2 for the purpose of preventing transverse movement of the boats. The corners of the boats would fit into the right angle shaped notch 52 of the upwardly extending portion 50 of such a support, which would prevent movement in either direction.

The same is true of the modification 56 of the end of the upwardly extending portion 54 of the support member shown in FIG. 4. This modification is essentially a pin which would mate with an appropriately located set of holes in the boats and thereby accomplish the same results as the modification depicted in FIG. 3.

The wheeled silicon nitride kiln furniture of the present invention need only have one set of rotatable wheels if one end of the base member thereof is attached to a mechanical means for moving the device in and out of the diffusion furnace.

What is claimed is:

1. in the manufacture of silicon semi-conductor components:

a device for conveying boats laden with silicon wafers into and out of a diffusion furnace, said device being comprised of a base member having attached thereto at least one pair of rotatable wheels and wheel attaching means, and at least one pair of supporting means for said silicon wafer laden boats, wherein said device is composed of 

2. The device of claim 1 wherein the base member, wheels and attaching means therefor, and the boat supporting means have a minimum specific gravity of 95 percent of the theoretical specific gravity of said silicon nitride-magnesium oxide composition. 